Comment 3 for bug 1395022

During the scan that's a custom SQL query to select all tracks without cover art. In this case the way the code works is you have to deserialize all the TrackPointers first to tell the background thread what to do -- so that's the issue here. We need a sort of lightweight TrackReference thing to truly be able to say "here's some work to do" to a background thread without doing a ton of heavyweight work.